1. Field
The present disclosure generally relates to optical devices. More specifically, the present disclosure relates to an optical de-multiplexer (de-MUX) that includes a sub-wavelength grating and a compact echelle grating.
2. Related Art
Integrated silicon photonics is a promising new technology that provides a number of advantages for chip-level communication, such as very high index contrast and compatibility with CMOS fabrication technology. Ongoing research into integrated silicon photonics is focusing on opportunities to provide: low latency, high bandwidth, high density, and low power consumption. To date, several key active elements, including silicon lasers, modulators, and photodetectors, have been realized in silicon using low-cost CMOS compatible processes. However, a wavelength filter (such as an optical de-MUX), which can be used in wavelength division multiplexing (WDM), has not been developed yet.
In a high-data-rate WDM application, an ideal wavelength filter has: low loss, low crosstalk, and a small footprint. In principle, a number of optical de-MUX designs can be integrated with CMOS circuits, including: Mach-Zehnder (MZ) lattice filters, ring resonators, arrayed waveguide gratings (AWG) and planar concave gratings (echelle gratings). Echelle gratings, which image and diffract optical signals, are particularly interesting because they can be designed to have: low optical crosstalk, fixed channel spacing, reduced tuning and monitoring requirements, and low on-chip optical loss.
However, echelle grating designs with high optical performance (such as low optical crosstalk and image aberrations) often have larger footprints. For example, image aberrations associated with grating facets far from the center of an echelle grating are one of the main contributors to optical crosstalk. As a consequence, wider entrance and exit apertures along the Rowland circle are usually required in order to reduce the input beam divergence in the free-propagation region of an echelle grating. In addition, smaller diffraction angles (less than 40°) are typically used to avoid illuminating grating facets further away from the center of the echelle grating. Given linear dispersion, these design choices result in a significantly longer focal length for the echelle grating and, thus, the device size or footprint is significantly increased.
Increasing the size of echelle gratings usually increases their manufacturing cost. In addition, larger echelle gratings are often excluded from size-sensitive optical links. Thus, echelle gratings are usually not used in systems that only include a few WDM channels (e.g., 4 or 8 carrier wavelengths).
Hence, what is needed is an optical de-MUX that does not suffer from the above-described problems.